Conventional portable lifting jacks typically comprise a wheeled frame, a lifting arm retractably received in the frame, a hydraulic pump for driving the lifting arm, and a handle connected to the hydraulic pump. A hydraulic cylinder is disposed at a front side of the hydraulic pump. A hydraulic shaft is extendable from and retractable in the hydraulic cylinder. A rear end of the hydraulic shaft is movably fitted in the hydraulic cylinder. A front end of the hydraulic cylinder is connected with the lifting arm via a link arm.
The hydraulic pump generally comprises a multipurpose block having at least one oil chamber and a plurality of oil chambers defined therein, a releasing device disposed in a release bore defined in a rear side of the multipurpose block, a piston cylinder disposed in a piston bore defined in the rear side of the multipurpose block, and a hydraulic cylinder securely connected with the multipurpose block at a threaded hole defined in a front side of the multipurpose block. When a user swings the handle manually, the lifting arm is driven by the hydraulic pump to move pivotally between a horizontal direction and an inclined direction. One prior art portable lifting jack is disclosed in U.S. Pat. No. 4,018,421.
One problem with the prior art lifting jacks is that, although the jack provides great mechanical advantage in lifting heavy loads, the lifting arm is actuated via the hydraulic cylinder, even when the jack is not loaded. Because of the high mechanical advantage, the lifting arm raises slowly even under no-load lifts.
There is a need for a portable lifting jack that provides a high mechanical advantage in lifting heavy loads yet can be raised quickly when not loaded. Accordingly, the present invention is hereby presented.